2 12 Of the Light manifested in Combustion. 



employed with facility in improving the art of illumi- 

 nation and the instruments that are employed in it. 



What vast sums are expended in dispelling the 

 obscurity of the night in every part of the world ; and 

 yet in what a deplorable state is the science which 

 ought to elucidate all the details of that important 

 operation ! 



How is it possible to labour with any prospect of 

 success to improve the methods employed in illumi- 

 nating our dwellings, as long as we remain so perfectly 

 ignorant respecting the nature of light as not even to 

 know with any degree of certainty whence it proceeds 

 or how it exists. 



After having meditated a long time on this interest- 

 ing subject, I have lately made a course of experiments 

 which I thought mischt lead to some useful discoveries. 

 But before I proceed to give an account of them it will 

 be necessary to mention a few alterations and improve- 

 ments which have been made in the apparatus (already 

 known) which I employ for measuring the intensity of 

 light. 



Instead of the rods divided into inches and tenths 

 of inches, which I formerly employed for measuring 

 the distances of the lights which are compared from 

 the middle of the field of the photometer, I now em- 

 ploy rods divided into degrees, which indicate directly 

 and without any computation the relative intensities of 

 those lights. 



These two rods, which are twelve feet in length, are 

 divided uniformly ; and they serve as a graduated scale 

 to the photometer. Their first division, which is at the 

 extremity of the rod nearest the photometer, is marked 

 io°, and it is placed at the distance of ten inches from 



